Power converters are used to convert direct current into alternating current and vice versa for many applications, for example for the coupling of electrical networks to speed-variable drives, for energy exchange between two electrical networks, for high-voltage direct current (HVDC) transmission, and the like. For this purpose, power converters are known in different circuit topologies and configurations. With progressive development of power semiconductor switches, newer converter topologies are constantly being sought and developed for increasingly higher power and voltage ranges. In the medium- and high-voltage ranges, what are known as multi-point power converters are being used increasingly in order to generate a number of voltage stages and in order to increase the voltages to higher levels reaching as far as HVDC transmission ranges.
A relatively new type of power converter topology is constituted by the modular multi-point power converter. This power converter has phase modules which consist of two branches connected in series with one another, wherein each branch is constructed from a number of identical power cells or sub-modules. Each power cell is formed by a bridge circuit, for example a full-bridge or H-bridge circuit having controllable power semiconductor switches and an internal capacitor for the temporary storage of electrical energy. Due to the modular structure, the power converter can be scaled individually for different powers and applications. One power converter branch can comprise up to or more than 300 power cells for example, which are connected in series with one another within a branch. In the case of a three-phase configuration of the power converter with six branches, 1800 cells or more can thus be provided.
A control device for power converters known from practice is illustrated in FIG. 1. A central control unit (CPU) 2 is connected to a plurality of power cells 3 for communication therewith, said cells being connected herein in series with one another. Each power cell 3 has a power semiconductor switch or a circuit, such as the above-mentioned H-bridge circuit, which is illustrated herein merely by a block 4.
The power cell 3 also contains what is known as a remote input/output (I/O) device 6, which forms the input and output device of the power cell. The remote I/O device 6 sends switching commands for the actuators or power semiconductor switches of the power cell and receives responses from sensors (not illustrated in greater detail herein) of the power cell 3, which sensors for example detect currents, voltages, temperature and other parameters in the power cells 3.
The remote I/O device 6 is connected to the control system via a communications module 7 of the remote I/O device 6, which, as illustrated in FIG. 1, has two communications connection ports 8A, 8B. In preferred implementations, the communications connections are provided with use of 100 megabit or gigabit Ethernet on the basis of fibre optic cables or copper cables.
In the currently known and used implementations of communications systems of this type, the remote I/O devices 6 are connected to one another in the line topology visible from FIG. 1. Herein, the central control unit CPU 2 is connected to a first communications port 8A of a remote I/O device 6 of a first power cell 3A. The power cell 3A is connected via its second communications port 8B to the first port 8A of the remote I/O device 6 of the second power cell 3B for communication therewith. The second power cell 3B is connected via its second communications port 8B to the remote I/O device 6 of a third power cell 3C, etc. Apart from the CPU and the remote I/O devices, no further devices are necessary herein for the communication. This is economical and simple for smaller systems and has proven in many systems to be reliable and to function well during use.
A line topology of this type of an industrial bus system comprising a master node, for example a CPU, and a plurality of slave nodes for operating high-power semiconductor switches, wherein the slave nodes are connected in series with one another via data cables, is known by way of example from DE 20 2013 207 826 B3. Here, the master node is arranged at an end of the series of slave nodes. An output data frame is sent in the output direction starting from the master node to the last slave node furthest away from the master node and passes through all slave nodes connected therebetween, whereas an input data frame, as response to the output data frame, is transmitted starting from the last slave node in the input direction towards the master node via all slave nodes disposed therebetween, which can add information to the input data frame.
There are various methods for communication in control systems of this type, as illustrated in FIG. 1. Generally, one transmit frame (Tx) and one receive frame (Rx) are transmitted per cycle for the transmission of real-time input and output data. Other methods are also possible which for example combine the transmission information and receiving information in a single frame or divide the information over more than a single transmit or receive frame. Regardless of the communications procedure, various addressing modes can be necessary and available in systems of this type. The addressing modes can be linked with various operating states of the system. For a higher number of remote I/O devices, however, it is preferred for no addressing information to be transmitted in the real-time communications frame. By way of example, in the case of 300 power cells 3 or remote I/O devices 6, as illustrated in FIG. 1, and two bytes per address information item, the entire address information would require a total of 600 bytes.
In a known communications method the addressing information in the transmit and receive frames is therefore dispensed with. Each unit knows at which point in the transmit frame it is to extract the information relevant for said unit (T-RIOn) and at which point in the receive frame it should insert response information (R-RIOn). This function, which is also referred to as communications control and management or CCM for short (Communication Control Manager), is integrated in the communications module 7 of the remote I/O device 6, as indicated in FIG. 1 by reference sign 9.
For large power converters, as mentioned in the introduction, the control power and operational capability are very important. For example, a control cycle time of 100 microseconds is assumed for the control power and includes the transmission of the transmit frame, the control process, the transmission of the receive frame, and a short break between two cycles. With a communications delay time of just 0.3 microseconds per remote I/O device and with 300 remote I/O devices, each transmit and receive frame would experience a total delay time of 90 microseconds. It is clear that with such a delay the control performance requirements in the case of a large power converter with use of a logical line topology can no longer be satisfied.
A good operational capability or operational readiness is achieved in systems of this type on the power electronics side by means of redundant power cells. The system design defines a number x of power cells, which can fail permissibly before the system has to be shut down. In the event of a loss of up to x power cells, the system should continue to function continuously without interruption and without the need for repairs. A redundant design of the number of power cells can be in conflict with a physical line typology of remote I/O devices.